The present invention relates to a coated cutting tool insert particularly for machining applications with requirements of high wear resistance and toughness behavior of the cutting edge. The coated cutting tool is particularly suitable for turning of steels such as stainless steels.
Cemented carbide cutting tools have been used in the machining of steels for decades. The tool is worn by different mechanisms such as abrasive and chemical wear, chipping or fracturing of the cutting edge limiting the tool life. Besides the chosen cutting data, cutting speed, feed rate and depth-of-cut, external cutting conditions such as off-centered workpiece position and casting skin on the workpiece material will also influence the wear of the tool. In addition, the cutting of stainless steel is considered to be a particularly difficult machining operation since in addition to the above mentioned wear mechanisms also adhesive wear is introduced. Adhesive wear is obtained when smearing material during the cutting operation continuously adheres to and tears off material from the cutting edge. A short tool life is therefore predominant when machining stainless steels. Furthermore, when cutting stainless steels at high cutting speeds, the thermal energy transferred to the cutting edge is considerable and the tool edge may partly or entirely plastically deform. This mode of deterioration of the cutting edge is known as plastic deformation wear. A high degree of plastic deformation resistance is in clear contrast to a requirement of substantial edge toughness.
Multilayer coatings comprising a number of coating layers of different materials which are laminated on the substrate, each of the first coating layers having a first thickness and each of the second coating layers having a second thickness, etc., are known. The layers should preferably have a different crystal structure and/or at least different lattice spacings. One example of such a technique is when the Al2O3 growth periodically is interrupted by a short TiN deposition process resulting in a (Al2O3+TiN)x multilayer structure see, e.g., Proceedings of the 12:th European CVD Conference page pr. 8-349. GB 2048960A discloses a multilayer coating with a multiplicity of alternating layers of 0.02 to 0.1 μm of hard materials of different compositions. U.S. Pat. No. 4,599,281 discloses a multilayer coating with alternating layers of an aluminium-boron mixed oxide and another oxide layer of e g Ti(C,N,O). Dreyer and Kolaska, Metals Society (Book 278), London, England (1982) 112-117 report an Al—O—N multilayer. In U.S. Pat. No. 4,984,940 Bryant et al. disclose a cutting insert composed of a cemented carbide substrate with 6.1-6.5 wt-% cobalt, a coating including a base layer of titanium carbonitride followed by a multilayered coating consisting of a plurality of alumina layers. A cemented carbide substrate with a coating comprising 6-8 alumina layers is also claimed in U.S. Pat. No. 5,700,569. WO 99/58738 describes a tool consisting of a hard wear resistant substrate and a CVD multilayer of about 50 layers. EP-A-1103635 claims a cutting tool of a cemented carbide substrate with 9.0-10.9 wt-% cobalt and a coating comprising a medium temperature CVD (MTCVD) deposited TiCN-layer and a multilayer composed of totally 7-41 layers of κ-alumina and TiN or Ti(C,N). EP-A-1245698, EP-A-1245700, EP-1209255, EP-1455003, SE 0401636-6 and SE 0401637-4 also relate to multilayer coatings.
Smoothing of coatings by mechanical post treatment in order to, e.g., minimize the friction between the tool and the workpiece is disclosed in EP-A-127416, EP-A-298729, EP-A-693574 and EP-A-683244.